This invention relates to an inspection apparatus based on nuclear magnetic resonance (NMR), and particularly to an inspection apparatus based on nuclear magnetic resonance and suitable for high sensitivity measurement using a superconducting quantum interference device (SQUID) flux meter.
A conventional NMR-based inspection apparatus using an SQUID flux meter is described in publication, for example, Applied Physics Letters, Vol. 47, No. 6, 1985, pp. 637-639. This apparatus operates in such a way that an electromagnetic wave for inducing the nuclear magnetic resonance is applied to an object using a known method and an NMR signal (also called "FID signal") from the object is received by a high-selectivity LC resonance circuit consisting of a receiving coil and a resonating capacitor soaked in liquid helium and detected sensitivity with an SQUID flux meter. The detected signal is amplified with an amplifier and then mixed down into a low-frequency signal using a signal provided by a high-frequency oscillator.
The above conventional technique is intended for a small flux below the quantum flux .phi..sub.o of the SQUID flux meter, however, the NMR signal is not necessarily below the quantum flux .phi..sub.o. Moreover, detecting a flux above the quantum flux .phi..sub.o necessitates a feedback circuit, which is difficult to operate in a high frequency region. The above publication does not recognize the problem that the apparatus eventually operates only in a low frequency region.